The field of the invention is the technique of measuring the velocity of object movements by means of an optical correlator.
More precisely, the present invention relates to an optical correlator comprising an optical system for producing an image of an object space, a spatial frequency filter arranged in or close by the image plane, and at least one photoelectric detector associated with the spatial frequency filter.
Optical correlators of this basic design are used for many different kinds of measurements of movements and distances. They, further, serve to monitor rooms, as for example disclosed in U.S. Pat. No. 3,972,021. The spatial frequency filters used for image correlation always have a constant periodic structure with regard to the measuring coordinate in a given measuring field. If an object moves in a direction perpendicular to the direction of observation of the correlator a measuring signal is obtained of which the frequency and/or the amplitude is proportional to the velocity of the object, as described in the copending patent application Ser. No. 623,559, of Heitmann et, al, filed Oct. 17, 1975 assigned to the same assignee as the present application.
It is, however, a disadvantage of such measurements that, if the object movement is not strictly perpendicular to the direction of observation, but extends in an inclined direction towards the observer, a measuring signal is obtained which creates the wrong impression of an increasing velocity, that increases the more the closer the object comes, depending on the perspective enlargement of the object and on the changing angular velocity with regard to the observer. In the co-pending patent application Ser. No. 671,104, of Leitz et. al. filed Mar. 29, 1976 also assigned to the assignee of the present application, it is already suggested to derive from the measured change in velocity a measurement signal that is indicative for the approach of the object.
A change of the perspective size of the object also occurs if the object moves directly in the direction of observation. If for measurement an optical correlator is used which includes a spatial frequency filter having a constant periodic structure here again the wrong impression of a change of the object velocity would be created. To overcome this disadvantage it is already suggested in the patent application Ser. No. 623,559 to use for the spatial frequency filter an arcuate division which is not linear with regard to adjacent division lines. Thereby a uniform and velocity proportional measuring signal may be obtained from an object movement in a direction perpendicular to the lines of division.
In many cases it is desired to measure object movements in different object spaces and in different object distances. Since a certain distinct surface portion of the spatial frequency filter is assigned to each object space by the imaging system of the optical correlator it has been suggested in patent application Ser. No. 671,104 to subdivide the spatial frequency filter into different portions having divisions of different grating constants and to assign different photoelectric detectors to each portion. The changes in the measuring signal caused by the perspective enlargement of the object image are also present in this case. During the evaluation of the measuring signals these changes are to be taken into account by a suitable selection of staggeredly in series located object spaces and by a suitable interrogation of the measuring signals. Difficulties do occur, however, with regard to the limitation of adjacent object spaces and also with regard to the discrimination of object movements during the transition from one object space into the other.
It is, therefore, an object of the present invention to make a better use of the correlation of perspective criteria of the object and its paths of travel during the imaging and the optical filtering in the optical correlator. It is a particular object to provide a simple manner of distinguishing in the image between relative movements in the direction of the perspective vanishing lines and relative movements transverse thereto.